In general, an image sensor is a semiconductor device for converting optical images to electrical signals, and is mainly classified as a charge coupled device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS) image sensor.
A unit pixel of a CMOS image sensor includes a photodiode and a MOS transistor, and a CMOS image sensor sequentially detects electric signals of pixels using a switching scheme and thus generates an image.
Compared to the CCD image sensor typically used in the art, a CMOS image sensor has a simple driving type, uses various scanning methods, minimizes the size of a product by processing signals in a single chip, reduces manufacturing costs using compatible CMOS technologies, and reduces power consumption.
The CMOS image sensor includes a photodiode region and a transistor region. The photodiode region converts a light signal to an electric signal, and the transistor region processes the electric signal.
The photodiode and the transistor are typically horizontally-arranged on a semiconductor substrate.
While the horizontal-type CMOS image sensor has advantages over the CCD image sensor, problems still exist.
In the horizontal-type CMOS image sensor, the photodiode is horizontally adjacent to the transistor on the substrate. Therefore, the photodiode region only encompasses a portion of the image sensor. As a result, the fill factor is reduced, and the resolution of the horizontal-type CMOS image sensor is limited.
In addition, with a horizontal-type CMOS image sensor, it is very difficult to optimize the process of simultaneously manufacturing the photodiode and the transistor. A shallow junction is required in order to reduce sheet resistance of rapid transistor processes, but the shallow junction may not be appropriate for the photodiode.
In addition, since additional on-chip functions are often added to the horizontal-type CMOS image sensor, the size of a unit pixel should be increased or decreased in order to maintain sensitivity of the horizontal-type CMOS image sensor.
When the size of the unit pixel is increased, the resolution of the horizontal-type CMOS image sensor is decreased. In addition, when the area of the photodiode is decreased, the sensitivity of the horizontal-type CMOS image sensor is decreased.